For the purposes of low-cost, mass production of certain circuits, such as low end microprocessors (also called microcontrollers) used in many toys and consumer products, it is sometimes more cost efficient to have a single standard circuit, than to have numerous different versions of the circuit. To make this possible, the standard circuit can be provided with a number of EPROM (electrically programmable read only memory) cells or EEPROM (electrically erasable and programmable read only memory) cells that are programmed after the circuit is manufactured to "configure" the circuit for a particular use. For instance, the bit values stored in the EPROM cells might determine the type of clock signal used by the circuit by connecting one of several subcircuits to an internal clock node. A second major use of microcontrollers with embedded EPROM cells is for emulation of other circuits (such as other families of integrated circuits) and for software development.
Of course, the use of EPROM and other non-volatile memory cells in microprocessors and microcontrollers is not, by itself, new. The present invention address the need for configuration circuits that: (a) are area-efficient to target market segments which are under tough price competition, (b) have low power consumption in order to enable battery operation, (c) have the ability to operate under a wide range of power supply voltages, such as a range of 6 volts to 2.5 volts, (d) have built-in testability features, (e) are capable of asynchronous operation, and (f) have alpha particle protection features. In the prior art, it has been difficult to meet all these criteria.
Even though the conventional sense amplifiers used in EPROM memory devices can be used to read the state of the EPROM configuration cells, such sense amplifiers are not cost effective in terms of the silicon chip area used. The reason for this is that conventional EPROM sense amplifiers in EPROM memory devices need to operate at very high speeds, and thus speed of operation is more important in such devices than area efficiency. For configuration circuits, speed of operation is not important. This is because in low-end microcontrollers, these registers typically need to be evaluated only when the chip is reset.
Under normal operating conditions, the configuration circuit should not have static power consumption. This is an important criteria for extending battery life.
The configuration registers should be capable of asynchronous operation, since there may not be a clock available during certain modes of operation, such as in programming mode, since the configuration register may need to be programmed first for configuring the appropriate clock circuitry.
Since the configuration registers may be used in a one-time programmable device, it is important to be able to test that the configuration cells are unprogrammed and that the sensing circuitry is operational at least for the default configuration register values.
The desirability for having alpha particle protection circuitry arises from the fact that configuration registers typically need to be evaluated only when the chip is reset. The present invention provides a latch strengthening feature to protect the latch in which sensed configuration values are stored from alpha particles that might otherwise change its state. Furthermore, the latch strengthening is provided in a space efficient manner.
A major market for one-time-programmable (OTP) parts are so-called "two battery" applications. That is, many OTP parts are used in toys and instruments that are powered by two serially arranged batteries. Typically, the battery operated instrument initially receives a power supply voltage of about 3 volts when the batteries are new. However, the voltage provided by the battery degrades with use. Many industrial microcontrollers are rated for operation at low supply voltages such as 2.5 volts. Industry standard specifications could require that these same parts also be operable at a power supply voltage of 6.0 volts.
It is difficult to make configuration registers which can operate over a wide range of supply voltages, such as 6 volts to 2.5 volts, while still satisfying low power consumption requirements and silicon area constraints. However, to sell configurable microcontrollers to different market segments, these voltage range, power consumption and silicon area criteria are very important. The main mason it is difficult to make configuration registers which can operate over a wide range of supply voltages is that the optimal resistance of the sense-amplifier devices for reading the EPROM cells is very different for 6.0 volt and 2.5 volt operation. In particular, at low voltages, if an unprogrammed EPROM cell is only partially turned on it draws very little current, making it hard to distinguish from a programmed cell. At high voltages, programmed EPROM cells are known to leak, which tend to make them look like unprogrammed cells.